1. Field of the Invention
The present invention relates to a sensor which senses the width of an aisle created in a mobile storage system when movable storage units are moved to open or close the aisles. The sensor provides a signal to the storage system controller to allow it to determine when to accelerate, decelerate, start and stop moving storage units when creating aisles for user access.
2. Description of the Prior Art
Mobile storage systems are well known in the prior art. These systems include a plurality of storage units, typically bookcases, wherein the end storage units are stationary and the intermediate units are mounted on rails and include an electric motor or other prime mover to propel the storage units along the rails. Typically, the storage units are arranged so that there is sufficient room for a single access aisle within the storage system and the mobile storage units are moved in order to select between which of the storage units the single access aisle is formed.
This general configuration is disclosed in U.S. Pat. No. 5,359,191 to Griesemer et al. entitled “Cross-Aisle Photo-Detector for Mobile Storage System Having a Light Detector Mounted to the Movable Storage Unit”.
It has been recognized in the prior art, such as in the above Griesemer et al. reference, that these mobile storage units can be very heavy, particular when full of books or other inventory, and that safety systems are required to prevent the closure of an access aisle between two storage units.
In order to control these units, limit switches have been used, such as disclosed in U.S. Pat. Nos. 5,005,923 and 5,044,703, both entitled “Limit Switch Apparatus Assembly for Mobile Storage Units” and issued to Dahnert. However, these references disclose apparatus which works only when an aisle is closing and cannot be used to decelerate or stop an opening aisle.
Similar deficiencies are encountered in the use physical limits switches on the lower carriage of a storage unit or on the overhead scissor arms and in the use of optical distance sensors. Moreover, optical distance sensors have minimum and maximum sensing limitation that must be accommodated when implementing the sensor. These sensors further typically have a non-linear response which must be compensated for in the controller and are subject to external influences such as ambient lighting and target variations. These sensors are also not suitable for sensing the limits of an opening aisle and do not provide hard-wired “end-of-travel” limits.
U.S. Pat. No. 5,670,778 entitled “Presence Detector for Mobile Storage Systems” issued on Sep. 23, 1997 to Smith (the inventor of the present application) discloses a safety system using an array of infrared light sources to detect the presence of a person or object within the aisle created by separated storage units.
Further improvements are sought wherein an electronic processor can monitor the distance between all storage units within the mobile storage system. Moreover, the use of a sensor with a linear output (that is, a signal directly proportional to the distance between adjacent storage units) is desired in order to simplify the required signal processing.